It's pretty nice on PC. Don't play it if you're just in it for the singleplayer though. It's probably very similar to the 360 version.
I will say that the multiplayer version offers a lot of replayability through unlockables and different game modes, but I don't believe you can play against XBox live players. Pretty sure that's reserved for Windows "Live" games or something. (It requires a monthly sub. too)
This is correct. A windows Live game and a windows Live gold account are needed to play crossplatform.
This is only offered on some games from Microsoft at the moment.